1. Technical Field
The present invention relates to an ophthalmological device for projecting femtosecond laser pulses. The present invention relates, in particular, to an ophthalmological device with an optical light projection module for projecting deflected femtosecond laser pulses onto a defined treatment surface into an image area extending from the optical axis of the light projection module.
2. Prior Art
Simple spherical lenses focus at the focal point only monochromatic light beams which run near the optical axis (paraxial beams). Beams running that are further removed from the optical axis are focused at another focal point. This most common image error is termed aperture error, or else spherical aberration. If the aim is to focus with a high numerical aperture in order to attain small spot sizes, it is possible to compensate these errors by aspheric lens shape, for example. This compensation provides no assistance for imaging locations lying off the optical axis. In addition, further aberrations such as coma, field curvature, astigmatism or distortions result during optical imaging. Furthermore, axial and lateral chromatic aberrations occur when polychromatic light is being used.
It is normal to use a combination of a number of spherical lenses with different refractive properties to compensate these errors. Diffractive optical elements are also used for this purpose in a few cases. In general, the outlay on correction, and thus the number of optical elements (for the most part lenses and mirrors) and materials which are used rises with the numerical aperture and the field size (size of the sharply focused image area). However, the diameter and the weight of the optical systems also rise substantially with the numerical aperture and the field size. When the focal plane is additionally to be adjusted, the outlay on equipment rises further. This relationship is of great importance particularly in the design of femtolaser systems. For example, femtolaser systems, which have pulse widths of typically 10 fs to 1000 fs (1 fs=10-15 s), require numerical apertures of greater than 0.2 in an ophthalmological application, since otherwise the material removal at depth (for example in the cornea) is not accurately defined, and optical breakthrough already comes about partially on structures lying above the focal plane (for example in the epithel). A further undesired phenomenon outside the focal plane are streaky structures (so-called “streaks”) in the tissue along the propagation direction of the laser beam. There are breakthroughs above and below the focal plane even in the case of systems with numerical apertures around 0.3. Highly corrected systems with a working area of 10 mm with a numerical aperture of 0.3 require as many as ten and more lenses with a diameter of about 100 mm. Raising the numerical aperture is possible in practice only in conjunction with a reduced image field (image area). In addition, in the case of phases of femtosecond laser pulses which are affected by aberrations, there is the problem that not all light beams come into focus at the same instant, because of different transient times. Particularly in the case of very short pulses, the maximum intensity at the focus is therefore reduced.
By way of example, Patent Application EP 1486185 describes such an ophthalmological device with an application head which can be used manually, and in this case the advantages of the small overall size and the low weight are bought at the expense of the disadvantage of a restricted image field.
Patent Application DE 10358927 describes a laser device for material machining by means of laser radiation, in particular for refractive surgery on the human eye by means of deflected femtosecond laser pulses. The device according to DE 10358927 comprises a pulse selection device which modifies selected laser pulses such that the modified pulses either no longer pass into the material to be machined at all, or can at least no longer produce an optical breakthrough there. The modification of the laser pulses comprises influencing parameters such as phase, amplitude, polarization, beam direction and field distribution over beam cross section (beam profile). By means of wavefront modification, in particular, the selected laser pulses are defocused such that the energy density no longer suffices for optical breakthroughs. The pulse selection, and thus the modification, perform, in particular, in a fashion synchronized with the deflection rate.